Passive Air Surveillance and the IAF
- In Dispatch
- 07:17 PM, Aug 05, 2015
- Vijainder K Thakur
UK's leading provider of air traffic control services, NATS, recently announced that several British companies have been testing the concept of using regular TV signals to track air traffic! [Source]
The concept was recently tested over London using a concept demonstrator developed by Thales UK, which used signals from the Crystal Palace transmitter in Bromley.
Researchers were able to simultaneously track up to 30 aircraft at altitudes of up to 10,000 feet with enough accuracy to ensure standard separation requirements for air traffic control. The capability of the systems could be expanded by deploying additional equipment.
Passive Radar
The concept of tracking aircraft by processing commercial broadcast and communications signals reflected by them is referred to as Passive Radar. Technically, a Passive Radar is a bistatic radar.
Typically radars transmit and receive signals through a single antenna. In a bistatic radar, transmitter and receiver antenna are geographically displaced but connected to each other through high speed data link. Radars can also be multistatic.
A Passive radar is a special case of bistatic radar because it uses a non-cooperative transmitter in combination with a cooperative receiver. The receiver and transmitter are not data linked, and work independently of each other.
When using cooperative radar transmitter and receiver, as in conventional (monostatic) or bistatic radar, the range of an object is inferred from the time interval between transmission of a radio pulse and the arrival back of reflected signals from the pulse. In a Passive Radar, the range of an object is inferred from the time interval between the signal directly received from a non-cooperating transmitter, and the signal reflected from an object.
A Passive Radar can determine the location, heading and speed of the object by measuring the bistatic Doppler shift of the echo and its direction of arrival.
Passive radar systems can exploit the following sources of illumination:
- Analog television signals
- FM radio signals
- Cellular phone base stations
- Digital audio broadcasting
- Digital video broadcasting
- Terrestrial High-definition television transmitters in North America
It's even possible to build a Passive Radar that exploits GPS satellites (GPS reflectometry) or satellite based commercial radio transmissions. However, satellite signals are considerably weaker than commercial terrestrial signals, and their reflections are more difficult to process.
Stealth and Bistatic Radars
Bistatic and multistatic radar architecture was initially used with Over the Horizon (OTH) early warning radars. Multiple receivers and transmitter mitigate the ambiguity in the signal path resulting from variations in the height and shape of the ionosphere.
Current research on multistatic radars is focused on their ability to counter Low Observable (LO) shaping of stealth fighters, by improving the signal to noise ratio of reflected signals. Multiple receivers well displaced from the radar transmitter are likely to see an enhanced radar cross section due to the geometry of radar reflections. Greater target cross section results in a stronger reflected signal. Also, multiple receivers are more difficult to jam or degrade, resulting in reduced reflected signal noise. Overall, signal to noise ratio improves.
Stealth and Passive Radar
Passive radars retain all the advantages of multistatic radars. In addition, they work with frequency bands that illuminate fighter sized stealth aircraft easily. For example, UHF band TV signals are strongly reflected by stealth fighters, unlike the S/X band signals from typical AD radars.
Passive Air Surveillance
It's likely that the inclusion of Passive Radars in future Integrated Air Defense System (IADS) would reduce the threat from adversary stealth aircraft. Well aware of the possibility, the IAF on December 2, 2014 released a RFI for development of a Passive Surveillance System (PSS).
The PSS would be a ground based system deployed in field areas for generation of 3D Air Situation Picture (ASP). It would work by detecting and processing chance RF spectrum emissions and EM reflections of other transmissions, in the vicinity of the airborne platforms.
The reference to 'chance RF spectrum emissions’ alludes to the use of Electronic Support Measures (ESM) for passive detection and tracking of aircraft.
Well known examples of such ESM systems are the Czech TAMARA and VERA systems and the Ukrainian Kolchuga system, which work by tracking adversary RF emissions such as those from radar, data link, or transponders.
Both ESM and Passive Radar systems can be used for air surveillance, but it’s worth remembering that Passive Radars work with reflected radio energy whereas ESM systems work with adversary transmissions. A well trained adversary exercising emission control (EMCON) is unlikely to be picked up by ESM, but will be picked up by Passive Radar.
The inclusion of Passive Radars in future IADS would result in stepped up adversary Electronic Attack (EA) to jam or disable commercial transmissions in target areas. The current thinking is that stealth and EA are complimentary and the combination will persist long into the future. Considering the advances made by China in stealth and electronics, the IAF needs to quickly firm up the timelines for induction of stealth and credible EA in the service.
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